Power limiter for discharge tubes



1945. w. VAN B. ROBERTS 2,358,780

POWER LIMITER FOR DISCHARGE TUBES Filed April 28, 1942 INVENTOR ATTORNEY Patented Feb. 6, 1945 UNITED STATES PATENT OFFICE I POWERLIll/HTEB. FOR DISCHARGE TUBES Walter van B. Roberts, Princeton, .N. J., assignor to Radio Corporation .of America, a corpora tion of Delaware Application April 28, 1942, Serial No. 440,883

14 Claims. 401. 250-27) f This invention relates to power supply circuits for amplifier tubes, especially tubes working at low efficiency such as class B stages or frequency multipliers, and provides means for limiting the power stages also whenever the dissipation of power in the supply circuits is not objectionable.

In the case of a purely dissipative loadthat is, a load in which all the input power is dissipated in heat, the power input may be limited to a desired value under all conditions of load variation while providing a normal input in the case of normal load resistance by employing the arrangement described in my United States Patent No. 2,218,925 and also in my article entitled "Foolproof screen feed, published in QST? for October 1940. In the case of an amplifier or frequency multiplier tube, however, conditions are somewhat difierent because not all of the input power is dissipated in the tube when the circuits are correctly adjusted.

My invention will now be described in more detail, reference being made to the accompanying drawing, in which:

Fig. 1 shows diagrammatically a circuit arrangement which exemplifies my invention;

Fig. 2 shows a modification of the circuit arrangement which also is comprehended in my invention; and

Fig. 3 shows another modification or my invention which is particularly suited to the requirements of a frequencymultiplier.

In Fig. 1, I show a conventional discharge tube circuit arrangement including a triode discharge tube I, the output circuit of which includes the direct current source 3, a resistor R, and a parallel-tuned circuit comprising inductance 5 and capacitor 1. The cathode of the tube I and also the negative terminal of the source 3 may be grounded if desired. In shunt with the source 3, I preferably insert a by-pass capacitor 9.

The circuit arrangement of Fig. 2 is similar to that of Fig. 1 with the exception that the output circuit for the tube l includes an additional resistor 2 connected between the cathode and ground. This resistor is preferably shunted by aby-pass con'denserd. r

Fig. 1 shows an amplifier or multiplier stage having a resistance R in serieswith the power supply voltage. A series resistance such as here shown is notof itself new and such resistances have been commonly :used during preliminary adjustments of circuits ,for safeguarding the .tube. However, once the circuit is adjusted the resistance has always been removed as it was :found to seriously reduce theoutput of the stage. In accordance with the present invention, however, this resistance is left :in for normal operation as it will be shown that by suitably ,increasing the voltage of the plate supply sourc above its normal value and try-properly choosing the value of resistance Rvthe output/of thestage may be maintained sufliciently near its normal value for many purposes while retaining the important advantage of safeguarding the tube against all contingencies at all times.

The operation and adjustment 1 the circuit of the invention may bestbe understood by reference to armathematical analysis. Referring to Fig. 1 let ustsuppose that in the absence of resistor R the .normal plate supply voltagezis E, the normal power input is P, and the dissipation W. In accordance with the invention the resistance R is now inserted and-the plate ,supply voltage is raisedto .a value mE where is To,

Jcoeflicient whose value will be determined -,later.

For the sake .of generality, let us suppose that during various circuit .misadjust-ments the effective plate circuit resistance of th tub m take onany value from zero to infinity. It may readily ,be demonstrated that the maximum input to the tube occurs when the effective tube resistance is equal to the resistance R and this maximum possibleinput is '(ntlj') Hence if it is desired that the tube .inputshou'ld never exceed W, we may equate (ms) 4R to W and thus by solving this equation i'or R, determine that the required value of R is mal efiective tube resistance E /P and the series is maximum when m is equal to V4W/P and when this value of m is employed the power input becomes equal to W. At the same time the value of the series resistance becomes equalto the nor mal effective tube resistance E /P.

Summarizing the results of the foregoing analysis it may be said that:

1. To obtain maximum output in the presence 7 of correct circuit adjustments while at the same timeproviding an upper limit of input equal to the dissipation of W of the tube, insert series resistance E /P and raise the plate supply voltaget'o Ex lW/P.

2. With the above provisions, the stage output is reduced from the normal value (that is, the value obtained prior to an installation of the safeguarding arrangements) in the ratio W/P.

r 3. From item 2 above it is seen that the reduction in output resulting from the use of the safeguarding'circuit of the present invention is less serious as the normal efficiency of the stage islower, so that the circuit is of particular advantage in low efficiency stages such as class B needed. Therefore, I do not believe my invention to be limited to the optimum adjustments of the circuit thereof, but only in accordance with the following claims.

I claim:

1. In a class B discharge tube stage, a tube and a source of anode potential therefor, a resistance connected in series with said source and said tube, said resistance being substantially equal to the impedance of said tube, thereby to limit the input power to said tube to a safe value and said time providing an upper limit of power input or linear radio frequency amplifiers or frequency v multiplier especially triplers, quadruplers, etc., or in low power stages where the power loss in the series resistor is not important and where the reduced output is still suflicient. Fig. 3 shows a preferred circuit arrangement for use in frequency multipliers.

Referring now .to Fig. 2, a series resistance 2 is shown connected between cathode and ground to provide bias for the stage. Preferably this resistance is made sufficient to provide the entire bias required. In the case of frequency multipliers where a high bias is desirable, it will usually be preferable to put the entire series resistance R between cathode and ground, as shown in Fig. 3.{'Itm'ight be thought that when the series resistance is arranged to provide high bias a lower total value of resistance would suflice for safeguarding the tube. This may be true in the case "of the commoner types of misadjustment of the stage, but to safeguard against the possibility of equal to the permissible dissipation within the tube.

3. In a frequency multiplier circuit arrangement, a discharge tube having at least a cathode and an anode and having a maximum safe wattage dissipation factor W, an output circuit connected between the cathode and anode, a direct current source having an adjusted terminal voltage equal to E /4W-P in said output circuit and means in said output circuit for limiting the input power to the maximum safe dissipation. of watts within said tube in the presence of either abnormal or normal circuit adjustments for said tube, said means including a resistor the value of which is equal to 7 of said source that would be required in the ab-- to said resistor and a value of Ex/ lW/P to said source, Where P represents the normal'power inputfor said tube and ,E represents the'normal voltage of said source that would be r'e'quired in the absence of said resistor.

WALTER VAN B. ROBERTS. 

